Dual pressure system for producing nitric acid and method of operating thereof

Abstract

A production plant for producing nitric acid at reduced power, the system being derived from a state-of-the art dual pressure nitric acid plant wherein the system further includes a first feature for splitting a tail gas stream into a first tail gas stream in fluid communication with compressed air and with an oxygen-rich gas and a second tail gas stream, and/or a feature for splitting a tail gas stream into a third tail gas stream in fluid communication with compressed air and with an oxygen-rich gas and a fourth tail gas stream. The production plant allows for reduction of power by the air compressor. A method for operating the system, the use of the system for performing the method, and a method for revamping a state-of-the-art dual pressure nitric acid plant into the system.

1 . A production plant for producing nitric acid at reduced power consumption and reduced emissions, comprising:
 an air compressor providing compressed air;   a supply for a first oxygen-rich gas, a mixing of the first oxygen-rich gas and of compressed air providing part of a first oxygen-containing gas;   a mixing apparatus, for mixing the first oxygen-containing gas with an ammonia gas stream, to produce an ammonia/oxygen-containing gas mixture;   an ammonia converter operable at a pressure equal to or higher than P1 and lower than P2, for oxidising ammonia in the ammonia/oxygen-containing gas mixture, to produce a NOx gas/steam mixture comprising water and nitric oxide;   a means for regulating a concentration of ammonia and/or of oxygen in the ammonia converter, including a means for controlling a flow of the first oxygen-rich gas in the oxygen-containing gas and/or a means for controlling a flow of the ammonia gas stream, for maintaining the oxygen to ammonia molar ratio inside the ammonia converter at a ratio of at least 1.2;   a first gas cooler/condenser downstream the ammonia converter, to produce an aqueous diluted nitric acid mixture and a gaseous NOx stream;   a NOx gas compressor for compressing the gaseous NOx stream, to produce a compressed NO x  gas stream at a pressure P2;   an absorption tower for absorbing NO x  gases from the compressed NO x  gas stream in water, to produce a stream of raw nitric acid-containing residual NO x  gas and a tail gas comprising NO x  gases, comprising an absorption tower tail gas outlet for evacuating the tail gas;   a heat exchange system located upstream the first gas cooler/condenser for heating a tail gas stream with the heat from the NO x  gas/steam mixture coming from the ammonia converter;   a second gas cooler/condenser for separating and condensing steam from the compressed NO x  gas stream before the stream is provided to the absorption tower;   a second oxygen-containing gas, having either:
 a) a pressure equal to or higher than P1 and up to P2, for supplying oxygen downstream the ammonia converter and upstream the NO x  gas compressor; or 
 b) a pressure higher than P2, for supplying oxygen to the compressed NO x  gas stream; 
   a means for controlling the flow of the second oxygen-containing gas such that a tail gas stream contains at least 0.5% by volume oxygen; and   a first pressure release means located downstream the heat exchange system, for expanding a tail gas stream, to produce a first expanded tail gas at a pressure equal to or higher than P1 and lower than P2, wherein the first pressure release means can at least partly power the NO x  gas compressor and/or the air compressor;   
       characterized in that the production plant further comprises:
 a first and/or a second means for splitting a gas stream, wherein
 (i) the first means for splitting is a means for splitting a tail gas stream into a first tail gas stream and a second tail gas stream, wherein the first tail gas stream has a pressure equal to or higher than P1 and up to P2 and is in fluid communication with the first oxygen-rich gas and compressed air, and wherein the mixing of compressed air, the first oxygen-rich gas and the first tail gas stream provides the first oxygen-containing gas, and 
 (ii) the second means for splitting is a means for splitting a tail gas stream into a third tail gas stream and a fourth tail gas stream, wherein the third tail gas stream has a pressure equal to or higher than P1 and up to P2 and is in fluid communication with compressed air and the first oxygen-rich gas, and wherein the mixing of the third tail gas, compressed air and the first oxygen-rich gas provides the second oxygen-containing gas, and wherein the second oxygen-containing gas is supplied downstream the ammonia converter and upstream the NO x  gas compressor;
 or 
 the second means for splitting is a means for splitting a tail gas stream into a third tail gas stream and a fourth gas stream, and wherein the third tail gas stream is in fluid communication with compressed air and the first oxygen-rich gas, and wherein the mixing of the third tail gas, compressed air and the first oxygen-rich gas; and the pressurization of the mixed third tail gas, compressed air and the first oxygen-rich gas in a means for pressurizing provide the second oxygen-containing gas at a pressure higher than P2, and wherein the second oxygen-containing gas is supplied downstream the NO x  gas compressor and upstream the absorption tower. 
 
 
 
     
     
         2 . The production plant according to  claim 1 , wherein the production plant further comprises a means for controlling the flow of the first and/or third tail gas stream. 
     
     
         3 . The production plant according to  claim 1 , wherein the production plant further comprises one or more of:
 a steam turbine, wherein the steam turbine can at least partly power the NO x  gas compressor and/or the air compressor;   a heat exchanger, for exchanging heat between the first expanded tail gas and a tail gas stream colder than the first expanded tail gas wherein the first expanded tail gas exits the heat exchanger at a temperature below 300° C., and wherein:
 the first expanded tail gas downstream the heat exchanger is in direct fluid communication with the first means for splitting; and/or 
 the tail gas stream which is colder than the first expanded tail gas is split into a third tail gas stream and a fourth tail gas stream; 
   a De-NO x  treatment unit; and   a second pressure release means for expanding the second tail gas stream to atmospheric pressure, to produce a second expanded tail gas.   
     
     
         4 . The production plant according to  claim 1 , further comprising a bleacher for bleaching the stream of raw nitric acid-containing residual NO x  gas, to provide a stream of bleached nitric acid, having an inlet in fluid communication with a high-pressure water electrolyzer supplying an oxygen-rich bleaching gas, and an outlet for off-gases in fluid communication with any gas stream downstream the ammonia converter and upstream the NO x  gas compressor if the bleacher operates at a pressure equal to or higher than P1 and up to equal to P2, or in fluid communication with any stream downstream the NO x  gas compressor and upstream the absorption tower if the bleacher operates at a pressure higher than P2, such that the supply for the second oxygen-containing gas comes at least partly from the off-gases. 
     
     
         5 . The production plant according to  claim 1 , further comprising a stream of a second oxygen-rich gas in direct fluid communication with any tail gas stream. 
     
     
         6 . The production plant according to  claim 4 , wherein the first oxygen-rich gas, the second oxygen-containing gas, the second oxygen-rich gas, the oxygen-rich bleaching gas and the off-gases are at least partly provided by a high-pressure water electrolyzer. 
     
     
         7 . A method for producing nitric acid at reduced power consumption and reduced emissions, in a production plant according to  claim 1 , comprising steps of:
 a) compressing air in the air compressor, thereby providing compressed air;   b) supplying compressed air obtained in step a) to the mixing apparatus;   c) supplying the ammonia gas stream to the mixing apparatus, thereby producing the ammonia/oxygen-containing gas mixture;   d) oxidising ammonia in the ammonia/oxygen-containing gas mixture in the ammonia converter at a pressure equal to or higher than P1 and lower than P2, thereby producing the gaseous NO x  gas/steam mixture comprising water and nitric oxide;   e) cooling the NO x  gas in the gaseous NO x  gas/steam mixture in the heat exchange system and in the first gas cooler/condenser, thereby producing an aqueous diluted nitric acid mixture and the gaseous NO x  stream;   f) compressing the gaseous NO x  stream in the NO x  gas compressor, thereby providing the pressurized NO x  compressed gas stream having a pressure P2;   g) absorbing the pressurized gaseous NO x  stream in the absorption tower, thereby providing the stream of raw nitric acid-containing residual NO x  gas and the tail gas comprising NO x  gases;   h) heating the tail gas in the heat exchange system, with the heat from the NO x  gas/steam mixture coming from the ammonia converter;   i) cooling the compressed NO x  gas stream in the second gas cooler/condenser, thereby providing the compressed NO x  gas stream having a temperature ranging from 20 to 60° C.; and   j) expanding at least part of the tail gas obtained in step h) in the first pressure release means, thereby providing the first expanded tail gas;   
       characterized in that the method further comprises the steps of:
 k) splitting a tail gas stream with a first means for splitting into a first tail gas stream and a second tail gas stream, and/or with a second means for splitting into a third tail gas stream and a fourth tail gas stream; 
 l) mixing the first tail gas stream with the first oxygen-rich gas and compressed air, thereby providing the first oxygen-containing gas, and/or mixing the third tail gas stream with compressed air and the first oxygen-rich gas, thereby providing the second oxygen-containing gas; 
 m) adjusting the flow of the first oxygen-rich gas being mixed in step l) or the flow of the ammonia gas stream to maintain the oxygen to ammonia molar ratio inside the ammonia converter to a ratio of at least 1.2; 
 n) supplying the first oxygen-containing gas to the mixing apparatus; 
 o) adjusting the flow of the second oxygen-containing gas such that a tail gas stream contains at least 0.5% by volume oxygen; and 
 p) supplying the second oxygen-containing gas at a pressure equal to or higher than P1 and up to P2 downstream the ammonia converter and upstream the NO x  gas compressor, or at a pressure higher than P2 downstream the NO x  gas compressor and upstream the absorption tower. 
 
     
     
         8 . The method according to  claim 7 , further comprising the step of:
 q) adjusting the flow of the first and/or the third tail gas stream.   
     
     
         9 . The method according to  claim 7 , wherein the first tail gas stream is mixed in step l), and wherein the first expanded tail gas is split in step k), and wherein the method further comprises the steps of:
 r) heating up, in the heat exchanger, the tail gas which is colder than the first expanded tail gas with the first expanded tail gas obtained in step j), thereby bringing the tail gas to be mixed in step l) to a temperature below 300° C.;   s) treating the tail gas stream heated in step r) in a De-NO x  treatment unit;   t) expanding the second tail gas stream in the second pressure release means, thereby providing the second expanded tail gas; and   u) recovering at least part of a heat energy generated in the ammonia converter in a steam turbine.   
     
     
         10 . The method according to  claim 7 , wherein the third tail gas stream is mixed in step l), and wherein the tail gas obtained in step g) is split in step k) into a third tail gas stream and a fourth tail gas stream. 
     
     
         11 . The method according to  claim 7 , further comprising the step of:
 v) bleaching the stream of raw nitric acid-containing residual NO x  gas obtained in step g) in a bleacher, thereby producing a stream of bleached nitric acid.   
     
     
         12 . The method according to  claim 7 , further comprising the step of:
 w) supplying the stream of the second oxygen-rich gas to a tail gas stream.   
     
     
         13 . The method according to  claim 7 , further comprising steps of:
 x) operating a high-pressure water electrolyzer, thereby producing pressurized oxygen-gas; and   y) providing, from the oxygen produced by the water electrolyzer in step x), at least part of the first oxygen-rich gas, the second oxygen-rich gas, the second oxygen-containing gas, an oxygen-rich bleaching gas and oxygen-rich off-gases.   
     
     
         14 . (canceled) 
     
     
         15 . A method for revamping an existing production plant for producing nitric acid, wherein the existing production plant comprises:
 an air compressor for providing a compressed air stream;   a mixing apparatus, for mixing compressed air stream with an ammonia gas stream, to produce an ammonia/oxygen-containing gas mixture;   an ammonia converter operable at a pressure equal to or higher than P1 and lower than P2, for oxidising ammonia in the ammonia/oxygen-containing gas mixture, to produce a NO x  gas/steam mixture comprising water and nitric oxide;   a first gas cooler/condenser, downstream the ammonia converter, to produce an aqueous diluted nitric acid mixture and a gaseous NO x  stream;   a NOx gas compressor for compressing the gaseous NOx stream, to produce a compressed NOx gas stream at a pressure P2;   an absorption tower for absorbing the NOx gases from the compressed NOx gas stream in water, to produce a stream of raw nitric acid-containing residual NO x  gas and a tail gas comprising NOx gases, comprising an absorption tower tail gas outlet for evacuating the tail gas;   a heat exchange system for heating a tail gas stream with the heat from the NO x  gas/steam mixture coming from the ammonia converter; a second gas cooler/condenser for separating and condensing steam from the compressed NO x  gas stream before it is absorbed in the absorption tower; and   first pressure release means for expanding a tail gas stream, to produce a first expanded tail gas at a pressure equal to or higher than P1 and lower than P2, wherein the first pressure release means can at least partly power the NO x  gas compressor;   
       into a production plant according to  claim 1 , wherein the method comprises steps of:
 introducing a supply for a first oxygen-rich gas in fluid communication with compressed air; 
 introducing a means for regulating the concentration of ammonia and/or of oxygen in the ammonia converter, including a means for controlling the flow of the first oxygen-rich gas in the oxygen-containing gas and/or a means for controlling the flow of the ammonia gas stream, for maintaining the oxygen to ammonia molar ratio inside the ammonia converter at a ratio of at least 1.2; 
 introducing a supply for a second oxygen-containing gas, having either: 
 (a) a pressure equal to or higher than P1 and up to P2, for supplying oxygen upstream the NO x  gas compressor; or 
 (b) a pressure higher than P2, for supplying oxygen to the compressed NO x  gas stream, 
 such that a tail gas stream contains at least 0.5% by volume oxygen; 
 introducing a first means for splitting and/or a second means for splitting a stream of tail gas downstream the absorption tower, wherein
 (i) the first means for splitting is a means for splitting a tail gas stream into a first tail gas stream and a second tail gas stream, and wherein the first tail gas stream has a pressure equal to or higher than P1 and up to P2 and is in fluid communication with the first oxygen-rich gas and compressed air, and wherein the mixing of compressed air, the first oxygen-rich gas and the first tail gas stream provides the first oxygen-containing gas, and 
 (ii) the second means for splitting is a means for splitting a tail gas stream into a third tail gas stream and a fourth tail gas stream, wherein the third tail gas stream has a pressure equal to or higher than P1 and up to P2 and is in fluid communication with compressed air and the first oxygen-rich gas, and wherein the mixing of the third tail gas, compressed air and the first oxygen-rich gas provides the second oxygen-containing gas, and wherein the second oxygen-containing gas is supplied downstream the ammonia converter and upstream the NO x  gas compressor;
 or 
 the second means for splitting is a means for splitting a tail gas stream into a third tail gas stream and a fourth gas stream, and wherein the third tail gas stream is in fluid communication with compressed air and the first oxygen-rich gas, and wherein the mixing of the third tail gas, compressed air and the first oxygen-rich gas and the pressurization of the mixed third tail gas, compressed air and the first oxygen-rich gas in a means for pressurizing provide the second oxygen-containing gas at a pressure higher than P2, and wherein the second oxygen-containing gas is supplied downstream the NO x  gas compressor and upstream the absorption tower. 
 
 
 
     
     
         16 . The production plant according to  claim 5 , wherein the stream of a second oxygen-rich gas is a stream of a pressurized oxygen-rich gas and the any tail gas stream is upstream the first pressure release means. 
     
     
         17 . The method according to  claim 7 , wherein tail gas is heated to a temperature ranging from 150 to 650° C. 
     
     
         18 . The method according to  claim 12 , wherein the stream of the second oxygen-rich gas is supplied as a stream of a pressurized oxygen-rich gas to the tail gas stream upstream the first pressure release means.